1. Field of the Invention
This invention relates to the field of Optical Coherence Tomography (OCT) imaging system and in particular, to a broadband discrete spectrum optical source and a method for enhancing the bandwidth of an OCT imaging system.
2. Description of the Related Arts
Optical Coherence Tomography (OCT) is a fast and accurate optical imaging technique, frequently used in producing high-resolution images for a variety of diagnostics and clinical applications. Currently, most common commercial application of OCT is primarily in the field of ophthalmology which provides eye clinicians the ability to quantify retinal nerve fiber layers by direct thickness measurements of the retina. As an imaging method, it offers diagnosis and care of eye diseases such as a glaucoma, macular degeneration, diabetic retinopathy (damage to retinal blood vessels) to name just a few. As the technology matures and becomes cost effective, applications of OCT may be expanded to other emerging applications in fields that includes but are not limited to, cardiology, dentistry, cancer diagnosis, glucose monitoring, and dermatology in near future.
Primarily, OCT is an interferometric technique in which the light from a broadband or a tunable source is split into a reference arm and a sensing arm of an interferometer. Light from the two arms are recombined and allowed to interfere at a detection system. The detected interferometric signal is processed in time domain or frequency domain, to obtain an image of a sample-for example, a retina or a fundus in ophthalmological diagnostics. An important condition to detect an interference signal is that the optical path difference between the two arms of the interferometer is shorter than the coherence length of the light source.
OCT can be configured to operate in a “time domain” (TD-OCT) imaging mode or in a Fourier-domain (FD-OCT) imaging mode. While TD-OCT system is accurate, some of the limitations are complexity, relatively low speed of mechanical scanning devices, and low source output power resulting in low imaging speed. These and other limitations are well documented in the co-pending U.S. patent application Ser. No. 13/111,047 and references cited therein, all of which in their entirety is being incorporated by reference here.
An FD-OCT system offers about 100 times more sensitivity and about 50-100 times faster image acquisition speed. Two equivalent FD-OCT configurations are currently being considered particularly for medical applications—a Swept Source (SS)—OCT configuration and a Spectral Domain (SD)-OCT configuration. With the same average source power, the performances of the SS-OCT system and SD-OCT system are identical regarding data acquisition and return loss. However, there are differences in system cost, complexity, speed, and depth resolution capability. Several advantages of an SD-OCT system over SS-OCT system described in other United States Patents, is summarized in the co-pending U.S. application Ser. No. 13/111,047. That disclosure is being incorporated by reference in its entirety.
A common drawback of the SD-OCT system described in the reference patents is that the broadband light sources such as, a Super Luminescent Diode (SLD) or an Amplified Spontaneous Emission (ASE) light source used in these prior art OCT systems exhibit low power density. Therefore power received at each detector of a detector array is relatively low, resulting in low image sensitivity. In order to improve image sensitivity, a longer signal integration time is required at the detector array thereby, limiting the speed of imaging. Imaging speed for the SD-OCT can be improved by increasing the signal level received at the detectors so as to reduce integration time at the detector.
The interference signal in a SD-OCT system is generated using light from a broadband source. Therefore a dispersive device such as a grating is necessary to separate the combined reference and sensing lights into its spectral components. Each spectral component can then be individually detected using a photo-detector array and addressed sequentially, to produce a digital spectrum (or a trace) of the interference signal. The detector signals from the detector array can be read out at a much faster rate than the source scanning rate in a SS-OCT system because source scanning is a mechanical process having its own limitations.
Despite its limitations, SD-OCT system is widely used in ophthalmology due to its simplicity and capability to produce quality 3D retinal images free from artifacts resulting from natural eye motion, at a reasonable cost. To produce spectrally smooth good quality image, the output power level of the order of a few mW (milliwatts) from a SLD's used currently in a SD-OCT system is insufficient. While data acquisition time for a single line scan (a so-called B-scan) is acceptable (less than a second), the time required to acquire hundreds of line scans to generate a quality 3D image is too long for patient's comfort.
In order to obtain better image quality and speed, higher optical power per detector element is necessary, while remaining within safe limits to prevent tissue damage (for example, retinal tissue). Higher power (10 to 100 times more photons per detector element) translates into shorter exposure time, deeper tissue penetration, and higher readout speed, frame rate, and sensitivity. The shorter exposure time would also enable the use of frame averaging to improve image quality in addition to reducing the total acquisition time of high-resolution 3-D images (also called “C-scans”). The preferred total exposure time for avoiding artifacts caused by natural eye motion and fixation drift is of the order of 3 seconds.
In the co-pending U.S. patent application Ser. No. 13/111,047, an OCT imaging system using a discrete spectrum high power optical source (also referred as light source or source, hereinafter) is disclosed. As disclosed therein, a high output power light source in the imaging system is configured using a gain medium such as semiconductor optical amplifier (SOA) or a SLD, placed within a reflective feedback optical cavity comprising two reflectors. High power output from the light source is achieved by designing the optical cavity such that the reflectivity of the front reflector is very low, of the order of 10−5 to 10−6; the reflectivity of the back reflector and gain are adjusted to obtain a desired output power. The output spectrum of such a light source exhibits a set of discrete emission peaks resembling teeth of a comb (a “COMB” source). In one variation of the discrete spectrum source, the back reflector is placed external to the gain medium at an adjustable distance from the gain medium.
In a preferred embodiment disclosed in the co-pending U.S. patent application Ser. No. 13/111,047, low facet reflectivity is achieved by tilting (preferably by about 6 degrees) the gain medium of a SOA waveguide with respect to the facet normal. In another preferred embodiment, the gain medium waveguide is perpendicular to the front facet and tilted with respect to the back facet of the gain medium device such that the waveguide is a bent or curved waveguide. The tilted or bent gain media reduces the natural back reflection from the waveguide facet and enables the extension of the cavity to a back reflector placed at some adjustable distance from the gain medium.
This invention provides a new broadband discrete spectrum light source comprising a gain medium placed within a feedback cavity that is designed to significantly extend the bandwidth of over a prior art discrete spectrum light source. When used in an OCT imaging system, the overall spectral profile and bandwidth of the discrete spectrum light source may be further enhanced by at least a factor of two, by applying signal processing methods to the detected interference signal. When used in a SD-OCT imaging system, the broadband discrete spectrum light source together with the signal processing method disclosed in this invention would improve depth resolution, sensitivity and speed of imaging over currently available SD-OCT imaging systems.